1) Field of the Invention
The present invention relates to a printed circuit board including a terminating structure for establishing impedance matching of a wiring line for transmitting an electric signal on the printed circuit board, and more particularly to a printed circuit board including a terminating structure suitable for establishing impedance matching of a wiring line on a printed circuit board on which a semiconductor unit (including a semiconductor device itself such as an LSI and a multi-chip module) having an internal wiring line having a resistance higher than a wiring line on the printed circuit board.
2) Description of the Related Art
Generally, an electronic apparatus such as a computer system includes a printed circuit board on which a semiconductor device such as an LSI or a multi-chip module (which may sometimes be hereinafter referred to simply as MCM) which includes a plurality of semiconductor devices is mounted. On a printed circuit board of the type mentioned, semiconductor devices and/or MCMs are interconnected by wiring line patterns (hereinafter referred to simply as wiring lines) serving as signal transmission lines for transmitting electric signals (digital signals). Further, in order to assure the quality of signals transmitted over the wiring lines, each of the wiring lines is terminated so that impedance matching may be established.
In the following, ordinary terminating systems are described with reference to FIGS. 7 to 10.
FIGS. 7 and 8 show systems wherein termination is effected on the reception side. The system shown in FIG. 7 is described first. Referring to FIG. 7, a transmission buffer (driving element) 100 and a reception buffer 200 are interconnected by a wiring line (signal transmission line, transmission line) 110. The input side of the reception buffer 200 (a terminal of the wiring line 110 adjacent the reception buffer 200) is grounded through a resistor 120 having a resistance value Rg and is connected to a power supply of a voltage V through another resistor 121 having a resistance value Rv.
In this instance, where the transmission line characteristic impedance of the wiring line 110 is represented by Zo and the output impedance of the transmission buffer (driving element) 100 is represented by Zout, the matching condition is given by EQU Zo=Rv//Rg
and the signal amplitude A is given by EQU A=[(Rv//Rg)/(Rv//Rg+Zout)].multidot.V
where EQU Rv//Rg=Rv.multidot.Rg/(Rv+Rg)
Meanwhile, in the system shown in FIG. 8, the resistor 121 in the system shown in FIG. 7 is omitted, and the input side of the reception buffer 200 (the terminal of the wiring line 110 adjacent the reception buffer 200) is grounded through a resistor 130 having a resistance value R.
The matching condition in this instance is given by EQU Zo=R
and the signal amplitude A is given by EQU A=[R/(R+Zout)].multidot.V
By connecting the resistors 120 and 121 or the resistor 130 which satisfies any of the matching conditions given above to the reception side to effect reception side termination, impedance matching can be established by any of the systems shown in FIGS. 7 and 8, and since signal reflection on the reception side can be suppressed, the signal waveform does not suffer from distortion and the quality thereof is assured.
It is to be noted that, while the system shown in FIG. 7 is preferable in terms of the balance in signal amplitude, since it requires two resistance parts, the system shown in FIG. 8 which requires only one resistance part is sometimes used as a simple form.
Meanwhile, FIG. 9 shows a system wherein termination is effected on the transmission side. Referring to FIG. 9, in the system shown, a resistor 140 having a resistance value Rd is interposed in series between a transmission buffer (driving element) 100 and a wiring line 110.
The matching condition in this instance is given by EQU Zo=Zout+Rd
and, the signal amplitude A is given by EQU A=V
By interposing the resistor 140 which satisfies the matching condition given above in series on the transmission side, impedance matching can be established also by the system shown in FIG. 9. In this instance, even if a signal is reflected on the reception side and comes to the transmission side, signal reflection on the transmission side can be suppressed, and consequently, the signal waveform does not suffer from distortion and the quality thereof is assured.
Further, FIG. 10 shows an ordinary terminating system in a case wherein bidirectional transmission is performed. Referring to FIG. 10, the terminating system shown in FIG. 7 is applied to termination on the opposite ends of a wiring line (signal transmission line) 110 through which bidirectional transmission is performed between a reception buffer 101 provided on a transmission buffer 100 side and a transmission buffer 201 provided on a reception buffer 200 side.
In particular, the output side of the transmission buffer 100 (the input side of the reception buffer 101) is grounded through a resistor 150 having a resistance value Rg1 and connected to a power supply of a voltage V through another resistor 151 having a resistance value Rv1. Meanwhile, the input side of the reception buffer 200 (the output side of the transmission buffer 201) is grounded through a resistor 160 having a resistance value Rg2 and connected to a power supply of the voltage V through another resistor 161 having a resistance value Rv2.
The matching condition in this instance is given by EQU Zo=Rv1//Rg1=Rv2//Rg2
and, the signal amplitude A is given by EQU A=[(Rv1//Rg1//Rv2//Rg2)/(Rv1//Rg1//Rv2//Rg2+Zout)].multidot.V
where EQU Rv1//Rg1//Rv2//Rg2=Rv1.multidot.Rg1.multidot.Rv2.multidot.Rg2/(Rv1.multidot .Rg1.multidot.Rv2+Rg1.multidot.Rv2.multidot.Rg2+Rv2.multidot.Rg2.multidot.R v1+Rg2.multidot.Rv1.multidot.Rg1)
By connecting the resistors 150, 151 and 160, 161 which satisfy the matching condition given above to the opposite sides of the wiring line 110 to effect termination, also where bidirectional transmission is effected, impedance matching can be established, and since signal reflection from one side of the wiring line 110 can be suppressed, the signal waveform does not suffer from distortion and the quality thereof is assured.
However, in the terminating systems shown in FIGS. 7, 8 and 10, the resistors 120 and 121, 130, 150, 151, 160 and 161 must be mounted, and consequently, if an increased number of such wiring lines (signal transmission lines) 110 are provided on a printed circuit board, then also the number of terminating resistances increases. This requires an increased physical space for mounting a large number of terminating resistances, resulting in reduction of the mounting efficiency and also in increase of the cost.
Further, since the signal amplitude on the reception side is dropped by the terminating resistance, in order to assure a sufficient signal amplitude, it is necessary to raise the driving capacity of the transmission buffer 100 and decrease the output impedance Zout of the transmission side. However, if the driving capacity of the transmission buffer 100 is raised, then since the size of the transmission buffer 100 increase and the area occupied by the transmission buffer 100 in the LSI increases, the number of signals which can be extracted from the single LSI decreases. The problem just described is more conspicuous particularly with such bidirectional transmission wherein the opposite ends of the wiring line 110 must be terminated as seen from FIG. 10.
Further, in the reception side terminating systems, since the wiring line 110 is connected to the ground or a power supply through a terminating resistor, even if a variation of a signal does not occur, current always flows from the transmission side. Consequently, the power dissipation of the transmission side increases as much. Further, as the power dissipation increases, also noise by a power supply which makes a factor of malfunction increases.
On the other hand, in the transmission side terminating system shown in FIG. 9, although such a situation that the signal amplitude decreases on the reception side or the power dissipation on the transmission side increases as in the terminating systems shown in FIGS. 7, 8 and 10 does not occur, it is still necessary to mount the resistor 140. Therefore, if the number of such wiring lines (signal transmission lines) 100 on the printed circuit board increases, then also the number of terminating resistors increases, resulting in increase of the physical space for mounting a large number of terminating resistors and in drop of the mounting efficiency and further in increase of the cost.
Also a terminating method is disclosed in Japanese Patent Laid-Open Application No. Heisei 7-73212 wherein a terminating resistor is not mounted as a discrete part but a wiring line itself has a designated resistance value so that matching of circuitry is established. According to this method, an impedance necessary to establish the matching of the circuitry is distributed uniformly to a wiring line between a signal outputting side element and a signal inputting side element. Consequently, the wiring line itself can be provided with a function as a terminating resistor, and the necessity for a discrete part for establishing matching of the circuitry is eliminated.
However, where an impedance necessary for establishing matching of circuitry is distributed uniformly to a wiring line, if the length of the wiring line is sufficiently short, then the wiring line can function as a terminating resistor, but as the wiring line length increases, that part of the resistance of the wiring line which serves as the terminating resistor is so distributed to the overall wiring line that, after all, the terminating resistance becomes lost, resulting in failure to establish matching of the circuitry and consequently in drop of the quality of the signal waveform. Examples of a waveform (result of a simulation) whose quality is dropped as a result of uniform distribution of an impedance to a wiring line are indicated by broken line curves in FIGS. 3 and 4. The waveforms will be hereinafter described in detail together with waveforms obtained by an apparatus according to a preferred embodiment of the present invention. In any event, the terminating method wherein a terminating resistance is distributed to a wiring line has a problem to be solved in that the length of a wiring line cannot be increased sufficiently since an increase of the length of the wiring line deteriorates the quality of the signal waveform compared with an alternative method wherein a terminating resistance is provided in a concentrated manner at an end of the wiring line.